1. Field of the Invention
The present invention relates to an active distance measuring apparatus.
2. Related Background Art
FIG. 6 shows a conventional active multi-point distance measuring apparatus (capable of measuring distances in a plurality of distance measuring regions in a screen). In FIG. 6, assume that the apparatus has three distance measuring regions on the right-hand side (R: Right), at the center (C: Center), and on the left-hand side (L: Left) in the screen and these regions are arranged adjacent to each other in the direction of a base line.
A camera main body 1 comprises a light projecting lens 2, a light receiving lens 3, and three light projecting elements 4R, 4C, and 4L, such as iREDs, and three light receiving elements 5R, 5C, and 5L, such as two-divided SPCs.
Light beams projected from the light projecting elements 4R, 4C, and 4L through the light projecting lens 2 are reflected by an object to be photographed (not shown) and focused on the light receiving elements 5R, 5C, and 5L, respectively, through the light receiving lens 3. The images on the light receiving elements 5R, 5C, and 5L change their positions in accordance with the distances to the object to be photographed. Therefore, distance information is calculated by integrating the far-distance output and the close-distance output from each light receiving element by using a known double integration scheme. More specifically, the far-distance output is integrated for a predetermined time T, and the sum of the far- and close-distance outputs is inversely integrated until it reaches the initial value. A time t required for this inverse integration is obtained, and the ratio of the predetermined time T to the time t is calculated, thereby calculating the distance information.
Compared to an apparatus having a distance measuring point only at the center of a screen, the above multi-point distance measuring apparatus has an advantage that the apparatus can prevent so-called off-centered focusing, such that in photography of two persons positioned alongside each other, these two persons become out-of-focus.
In the multi-point distance measuring apparatus of the above sort, light projected from the light projecting element 4C is incident on the light receiving element 5L when an object to be photographed is at a close distance. Therefore, taking advantage of this phenomenon, the apparatus checks whether the output from the light receiving element 5L is present, and, in accordance with the check result, i.e., if the apparatus determines that the output from the light receiving element 5L is present, it provides alarm (close alarm) indicating that an object to be photographed is present at a close alarm distance.
The above conventional apparatus, however, can check only whether an object to be photographed is present at a close alarm distance. This makes it impossible to apply the multi-point distance measuring apparatus of this type to a camera with a so-called macro photographing function, which is required to accurately measure a distance up to a very close distance (0.45 m) in addition to normal photographable distances (6 m to 0.6 m), as in the case of recent cameras.
The conventional distance measuring apparatuses for performing distance measuring for macro regions have the following problems.
1) Since the depth of field in macro photographing is small, a probability that a distance measured by one-time distance measuring, as in the conventional apparatuses, is within the depth of field is low. Therefore, distance measuring must be performed a plurality of times by turning a switch SW1 on and off for each measuring.
2) No conventional apparatuses perform an in-focus or out-out-focus indication in macro measuring. In addition, in order to realize this function, normally three indicators are required to indicate conditions of in-focus, close-side, and far-side, resulting in an increase in cost.